Burner Pilot With Virtual Spinner

ABSTRACT

A method and apparatus for preheating a furnace during a warm-up phase of furnace operation. The furnace has main burners with a tubular fuel supply surrounded by a main combustion air duct defining an annular space between the supply and the duct that extends in an axial direction of the main burner. A pilot nozzle in the annular space extends in an axial direction of the burner towards an interior of the furnace and discharges readily ignitable fluid fuel jets through orifices in the nozzle toward the interior of the furnace. Combustion air from the duct is directed past the nozzle and is mixed with the fuel discharged from the orifices to form an ignitable mixture that is ignited to form the furnace heating pilot flame downstream of the nozzle. The flame is stabilized and anchored to the pilot nozzle by recirculating portions of the flame and its constituents from the furnace interior back towards the nozzle by protecting the air passing through the primary ignition zone from being directly affected by air flowing through the main combustion air conduit, diverging the fuel jets relative to the axial direction by an angle between about 20° to 80°, and giving the fuel jets a tangential directional component relative to the axial direction to spin the flame about the axis of the pilot.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims the priority of Provisional Patent Application No. 60/967,915 filed on Sep. 6, 2007, the disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to warming up large utility-type furnaces, such as are used, for example, to generate steam for major electrical power generating plants, particularly but not limited to coal-fired plants, prior to the start-up of such furnaces to commence their production phase of operation.

Such large furnaces can have the size of a large building, and they often employ dozens of spaced-apart burners to provide the needed heat for generating large amounts of electrical power. These furnaces are fired with all types of fuels, including, but not limited to, oil, gas, coal, bio-mass, etc. The furnaces require an initial heat-up to bring their interior to the required operating temperature at which all burners can be fired fully without causing flameouts, generating large amounts of smoke and other pollutants, potentially damaging portions of the furnace due to excessive heat differentials, and the like.

Operators of such furnaces desire that the warm-up period is as short as possible because during warm-up phase, expensive fuel is consumed without generating any power. In the past, so-called warm-up guns were preferred over smaller pilot burners, especially for coal-fired furnaces, to generate sufficient amounts of heat over a relatively short period of time so that the production phase of the furnace can commence as soon as possible.

While natural gas and oil can be quickly ignited and do not require long warm-up periods, coal-fired furnaces encounter the problem of having to heat the furnace sufficiently so that the large mass of coal consumed by the furnace during its production phase can be ignited and will burn cleanly and completely without emitting excessive pollutants. Conventional burner pilot lights that are also used for the main burner ignition and flame stabilization either have too low a heat output to accomplish the required warm-up over a desired period of time, or require an additional air supply besides the air passing through the main burners, which makes their installation more complicated and expensive.

It is not normally feasible to increase the size of the pilot burners to enhance their heat output because pilots are arranged in the relatively small annular passage between the main burner and the surrounding combustion air duct. This limits the size of pilots, for most industrial installations, to no more than about four to five inches in diameter. With such size limitations, the maximum heat output of pilots not using additional air supply is typically limited to around 3 to 7 million BTUs per hour.

Increasing the fuel flow rate through the pilots beyond that range results first in unstable operation sensitive to the regime of air flow through the main burner. The operation becomes unsatisfactory because the resulting high velocity fuel jets can snuff out the pilot flame.

Further, the high capacity pilots need to be located at the main burner discharge end to generate the flame in the furnace interior and prevent the flame from burning the main burner. At the same time the pilots need to be protected from the main burner flame. In order to meet both these requirements, such installations often require complicated mechanisms to subsequently retract the pilot rearwardly out of the heat and away from fuel particles into the combustion air duct, which are costly, require much maintenance, and are subject to early failures.

Separate warm-up guns not tasked with the main burner ignition were therefore widely employed for warming up furnaces. Although such guns are capable of generating large amounts of heat and, therefore, can significantly shorten the warm-up period for even coal-fired utility-type furnaces, they require their own combustion air supplies as well as relatively complicated installations including their own piping, fans, motors, controls, gun retracting mechanism and the like, all of which make separate warm-up guns expensive to install and maintain.

BRIEF SUMMARY OF THE INVENTION

The present invention improves the manner in which large furnaces, such as are used for commercial power generation, and particularly coal-fired furnaces, are preheated during the initial start-up phase of furnace operation when the interior furnace temperature must be raised sufficiently to commence the production firing of the furnace. This is accomplished in accordance with the present invention by placing a pilot burner (hereafter typically “pilot”) with a much higher heat output in the same limited annular passage between the fuel, e.g. coal supply conduit, and the surrounding combustion air supply duct, where prior art burners have been commonly placed in the past. The pilot of the present invention is operated with combustion air for the main burner and eliminates the need for a separate air supply for the pilot. The pilot flame is ignited and stabilized by injecting a portion of gaseous fuel delivered to the pilot in a spinning pattern that creates intense recirculation and mixing of the discharged pilot fuel with appropriate amounts of air passing through the main burner around the pilot.

Such pilots can provide a heat output in the range between about 4 to 50 million BTU per hour, which is much higher than the heat output that could be achieved with prior art pilots operating without the additional air, and assures a rapid heat-up of the furnace and a relative quick start-up of its production phase. Substantial amounts of fuel otherwise used by the pilot without producing useable steam or electricity are thereby saved.

The present invention provides both a method and an apparatus for preheating furnaces, particularly large utility-type furnaces that have many burners which often operate with difficult-to-ignite coal during the warm-up phase of furnace operation. Generally speaking, this involves a main production burner that includes a first conduit for directing a fuel, for example coal, into an interior of a furnace. An air duct surrounds the coal conduit to define an annular combustion air passage into the furnace where the coal and combustion air are mixed and ignited during the production phase of furnace operation.

A pilot nozzle is positioned in the air passage of the main burner so that a downstream end of the nozzle is proximate the downstream end of the burner. The nozzle is surrounded by a tubular hood which has an open downstream end proximate the downstream end of the nozzle and an upstream end. Air flowing through the main burner passage for air is prevented from directly entering the hood by placing a flow inhibitor, such as a plate, over the upstream end of the hood while permitting air to enter the hood via a gap formed between the hood and the plate.

A relatively lesser portion of a pressurized fluid fuel, e.g. a gas, is flowed through igniter orifices in the nozzle located inside the tubular hood at a rate commensurate with the amount that can be burned within the limited space inside the tubular hood. The fuel gas jets inside the hood are directed at angles that facilitate entrainment of air through the hood. A major portion of the pilot gas is discharged from main pilot orifices—a plurality of spaced-apart pilot orifices in a downstream end portion of the nozzle immediately adjacent to the hood. The fuel jets from the main pilot orifices are oriented so that the emitted fuel jets angularly diverge in the downstream direction and have a tangential flow component relative to the longitudinal axis of the pilot nozzle. As fuel jets discharging from the main pilot orifices pass in the vicinity of the hood downstream end, they also facilitate the flow of air through the hood.

Fuel emitted by the igniter orifices inside the tubular hood is mixed with air passing through the hood and is ignited to generate an igniter flame that propagates past the downstream end of the hood. The igniter flame in turn ignites the mixture of fuel from the main pilot orifices and air passing through the main burner to generate a pilot flame that extends into and heats the furnace interior. Portions of the pilot flame and its constituent gases recirculate from the furnace interior rearwardly towards the nozzle while the flame as a whole spins relative to the nozzle axis to maintain a stable pilot flame.

One reason why placing high heat output pilot burners inside the combustion air duct of the main burner has heretofore been unsuccessful was that the volume of air flowing through the duct may vary substantially so that the pilot fuel flowing with a fixed rate often fails to ignite, or to maintain the flame, due to unfavorable fuel-to-air ratios, unless the pilot has its own air supply and controls. This is overcome by the present invention because the amount of air entering the hood is substantially proportional to the fuel delivered inside the hood through the igniter orifices and only to a small degree affected by the amount of flow through the duct as its upstream flow inhibitor effectively shields the pilot fuel from the effect of the high velocity air flowing through the duct. The hood forms a small combustion chamber where a relatively minor amount of the pilot fuel is initially ignited to form the igniter flame which propagates in a downstream direction past the downstream end of the hood, where the major portion of the pilot fuel is discharged via the appropriately positioned and oriented pilot orifices.

The hood, including the earlier mentioned flow diverter, also effectively shields sensitive components like the spark electrode inside the hood from the heat of the main pilot flame and the furnace, which allows operating the burner without having to retract the pilot into the burner.

In addition, to maintain a pilot flame, it must be stable and remain anchored to the nozzle. High heat output pilots require high fuel velocities through the burner orifices of as much as 500-1500 ft./sec. Such high fuel jet velocities lead to undesirable flame instabilities which are significantly reduced or entirely eliminated in accordance with the present invention by imparting a spin to the pilot flame downstream of the nozzle that facilitates establishing a recirculating flow downstream of the nozzle. To attain such a spin, the axes of the pilot orifices are tangentially offset relative to the pilot axis as described in more detail below. The tangential flow component of the jets provides the spinning results obtained with common prior art burners by placing relatively large spinners around the nozzles that cannot be applied here due to the earlier mentioned space limitations.

Another important advantage of the present invention is that the amount of air entering the interior of the hood automatically adjusts itself to the amount of fuel emitted by the igniter orifices inside the hood because as the volume of emitted fuel varies, its speed varies correspondingly, which in turn lowers or raises the fuel pressure inside the hood inversely to the velocity of the fuel emitted from the igniter orifices. With the lowered pressure, more air from the air duct is aspirated into the hood interior so that an approximate stoichiometric balance between the fuel and the air in the hood is maintained. This assures an uninterrupted igniter flame to maintain the main pilot flame even in the event of a temporary flameout. The amount of air drawn into the hood is correspondingly lowered as less fuel is emitted from the igniter orifices of the nozzle and the pressure inside the hood rises correspondingly.

Thus, the pilot burner of the present invention is relatively inexpensive because it has no moving parts and needs no internal or external controls.

A further advantage attained with the present invention is that the pilot burner is shielded from the high temperature and abrasive/corrosive/contaminating influences of the gases, dust and particles on the furnace interior because the pilot is located inside the air duct, which reduces maintenance costs and prolongs the life of the burner. Still further, since the pilot burner of the present invention requires no external controls, separate air supply lines and the like, it can be made relatively larger in the limited space available in the air ducts of industrial burners. This in turn makes it possible to increase the heat output of the burner and thereby shorten the warm-up period for the furnace, all of which reduces operating costs for the furnace warm-up and pilot burner maintenance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a large, e.g. utility-type, furnace arrangement for driving a steam turbine as used in large electric power generating plants;

FIG. 2 is a schematic, cross-sectional view through a burner, including a high heat-output pilot constructed in accordance with the present invention for installation in the furnace shown in FIG. 1;

FIGS. 3A and 3B are sectional views of the pilot of the present invention;

FIG. 4 is a schematic front elevational view of the main burner and the pilot shown in FIG. 1;

FIG. 5 schematically illustrates the formation of a pilot flame recirculation zone in accordance with the present invention;

FIG. 6 is an end view of an air flow restrictor plate of the pilot shown in FIGS. 3A and 3B;

FIG. 7 is an end view of an air flow straightener that prevents combustion air from flowing directly into a hood surrounding the pilot; and

FIGS. 8A and 8B are end and side elevational views, respectively, of the nozzle of the pilot shown in FIGS. 3A and 3B.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically illustrates a large power generation installation, as is commonly used, for example, by public utility companies for generating electricity for the public. The installation has at least one large, utility-type furnace 2 and many, typically dozens, of main production burners 4 which extend through at least one wall 6 of the furnace into its interior 8. Such furnaces can be and are fired with all kinds of fuels, with oil, coal and natural gas being the most common. The present invention has particular (but not sole) applicability to firing the furnaces with coal which is typically ground to fine powder or dust. As is well known, the heat generated by the fuel on the interior of the furnace generates steam 10 that can be used to drive a turbine 12 which may be connected, for example, to an electric generator (not shown). Exhaust gas from the furnace is released to the atmosphere through a stack 14, typically (but in many of the areas of the world not necessarily) after having been appropriately cleaned and/or scrubbed to limit atmospheric pollution.

FIG. 2 schematically shows the use of the present invention with a main production burner 4 mounted on and operatively extending through one of the furnace walls 6 and constructed to burn coal, typically finely ground or pulverized coal. It has a coal supply source 18 and a coal supply conduit 20 in which powdered, pulverized or the like coal flows in a downstream direction to a discharge end 22, which may include a spinner or diverter 24 for discharging the coal via an outwardly flared burner throat 26 in furnace wall 6 into the interior 8 of the furnace. Main burner 4 further has a combustion air supply duct 32 which concentrically surrounds coal supply conduit 20 to form an annular combustion air passage 34 between the coal supply conduit and combustion air duct. During operation, combustion air needed to burn the coal (or other fuel) is discharged from the downstream end 22 of the burner into the furnace interior. The main burner may include a supplemental fuel supply tube 28 which runs coaxially through (the horizontal portion of) the main burner and has a fuel discharge end cap 30 that can be used to provide additional heat from firing oil or gas, for example during peak demand periods for electricity when more heat output is needed.

The construction and operation of such main burners is well known to those of ordinary skill in the art and, therefore, is not further described herein.

Burner installation 4 includes a pilot burner 36 constructed in accordance with the present invention to initiate combustion in the furnace interior and, during a start-up phase of operation of the furnace, to warm up the furnace interior until main burners 16 can be fired after the furnace interior has reached the required temperature for maintaining a stable and complete combustion of the coal (or other fuel). The pilot has a feed tube 38 through which a fluid fuel, such as natural gas for example, is supplied from an appropriate source (not shown) to a pilot nozzle 40. The nozzle is surrounded by a tubular shield or hood 42, the ends of which are open, and an igniter, e.g. an electrical spark igniter 44, is provided for igniting the fuel, as is further described below.

FIGS. 3A, B and 4 show the pilot burner of the present invention in greater detail. Nozzle 40 includes and is attached to a downstream end of feed tube 38, has a discharge (or downstream) end 50, and has a plurality of pilot fuel discharge orifices 52 from which pilot fuel jets flow. The pilot fuel jets are discharged at an oblique angle relative to the longitudinal axis of the pilot burner, and they are additionally tangential to the axis of the pilot as is further described below.

Tubular hood 42 has open upstream and downstream ends 66, 68, respectively. A flow straightener and conditioner 70 (shown also in FIG. 7) is positioned inside the upstream end of the hood and extends some distance into the hood. A fuel feed tube 38 and an igniter support pipe 76, respectively, extend into the hood 42. The flow straightener includes a plurality of ribs 80 placed between the hood 42 and the fuel feed pipe 38 parallel to the igniter burner axis 96. The ribs define multiple flow straightening passages 82 that extend in an axial direction of the pilot. Air flowing between the ribs 80 becomes better oriented in the axial direction of the pilot, a feature which is particularly useful in instances when air flowing through the passage 34 is at an oblique angle relative to the pilot axis.

Pilot 36 is further fitted with a damper plate 84 (also shown in FIG. 6) which is spaced apart from the upstream end 66 of hood 42. The damper includes a tubular hub 86 that surrounds pilot fuel feed tube 38 and is slidably movable therealong. Opposite hub 86 is a U-shaped cutout 88 through which igniter support pipe 76 extends.

The axial position of damper plate 84 relative to the upstream end of the hood can be adjusted by moving the plate along fuel supply tube 38 of the pilot burner to vary the width of a gap 90 between the upstream end of the hood and the damper plate to accommodate specific characteristics of the fuel and provide a range of air flows through the burner 32.

The downstream end of igniter support pipe 76 ends at a bluff body 92 (FIGS. 3A, B) attached to the inside of the tubular hood 42. An electronic igniter 94 is placed inside the support tube 76 end about flush with the bluff body 92. On the side facing the flow, the bluff body 92 is shaped with a slope 93 that eliminates stagnation areas to the flow upstream of the igniter 94. Suitable hardware and wiring (not shown) for the electronic igniter extends through the igniter pipe 76 to an igniter control (not shown).

In a presently preferred embodiment of the invention, pilot nozzle 40 is configured as a cap attached to the downstream end of fuel feed tube 38 and has a multiplicity of fuel discharge orifices 52 arranged in a plurality of, e.g. two, rows 52A, 52B that are spaced apart in the axial direction of the nozzle, as illustrated in FIG. 3B. Each orifice diverges in a downstream direction relative to the pilot burner axis 96 by an angle α (shown in FIG. 8B) in a range between about 20° to 80°, preferably in a range between about 35° to 75° and in the presently preferred embodiment at an angle of about 60°.

In addition, each orifice 52 is arranged so that its center line 98 is offset relative to a radius line 100 with its origin at the center 96 of the nozzle so that each orifice is also tangential relative to this center, as is illustrated in FIG. 8A. This causes the fuel flow and flame in the wake of the nozzle 50 to spin in a manner analogous to a conventional spinner and anchors the flame to the pilot in spite of the high velocity fuel jets emitted from the orifices.

In a presently preferred embodiment, the pilot nozzle 40 additionally includes relatively small-diameter center holes 102. In use, gas flows through the center holes which cools the nozzle center.

Referring to FIG. 4, pilot nozzle 40 and igniter 44 are offset relative to the axis of tubular hood 42 so that the pilot nozzle is adjacent one side of hood 42, to thereby define an enlarged space 104 between the periphery of the pilot nozzle and the opposite wall of the hood where an initial igniter flame is generated, as is further described below. Arrows 106 in FIG. 4 illustrate the tangential positioning and orientation of fuel jets 53 (shown in FIG. 5).

Turning to the operation of pilot 36 for starting up a cool furnace, combustion air flows through annular passage 34 of burner 32 in a downstream direction past tubular hood 42 and then into the furnace interior 8. The gas for the pilot is flowed through feed tube 38 to orifices 46 and pilot nozzle 40. Sizing of the orifices 46 is such that a relatively minor portion of the fuel exits through igniter orifices 46 in the feed tube 38 which are oriented to direct resulting fuel jets into the enlarged space 104 inside the hood and in the vicinity of igniter 44. At the same time, air from annular passage 34 of the main burner enters the interior of hood 42 via gap 90 between the upstream end of the hood and damper plate 84. Flow straightener 70 straightens out the incoming air so that it flows generally in the direction of the pilot axis and becomes mixed with fuel from igniter orifices 46. The resulting mixture is ignited by spark igniter 94 to form an igniter flame 47 in the enlarged space 104 which propagates in a downstream direction past downstream end 68 of the hood, as is illustrated in FIG. 5.

The bulk of the fuel for preheating the furnace is ejected through orifices 52 in nozzle 40 as gas jets 53 which diverge outwardly in the downstream direction so that the ejected fuel becomes mixed with combustion air that flows through the annular passage 34 of the main burner. This mixture is ignited by the igniter flame 47 exiting from the downstream end of the hood which maintains the main pilot flame 54.

The amount of combustion air typically flowing through the annular passage 34 depends on the operational needs of the regime and is substantially independent of the pilot burner operation. The rate at which fuel is needed for the pilot also may be changed for operational reasons. To maintain the igniter flame 47, the amount of air fed to the burner must reflect the amount of fuel ejected by the igniter orifices to maintain an overall flammable mixture inside the hood 42 on the downstream part of bluff body 92.

To properly control the flow of air into hood 42, damper plate 84 blocks combustion air flowing through annular passage 34 directly into the hood. Instead, combustion air must first flow from the annular passage in a radial direction (relative to hood 42) through gap 90 and is then redirected past flow straightener 70 into the interior of the hood, thus minimizing the effects of air flow velocity through the passage 34 onto the amount of air flow entering the hood 42. The axial position of damper plate 84 relative to the upstream hood end can be adjusted by moving the plate, including its flange 86, along feed tube 38 to set the proper width for gap 90 to permit a sufficient air flow into the hood while preventing variations in the combustion air flow in the annular passage from materially affecting the air flow rate through the hood.

In use, the position of the damper plate is not normally changed. The air intake via gap 90 into the hood is nevertheless automatically varied as a function of the gas flow rate through igniter orifices 46 because as the gas velocity through the igniter orifices increases or decreases, the pressure inside the hood changes inversely to the pressure changes. An increase in the gas velocity through the igniter orifices lowers the pressure in the hood, which causes an increase in the air flow rate through gap 90 into the hood and vice versa. This air flow variation occurs automatically and requires no controls of any type.

Accordingly, the pilot burner of the present invention is self-regulating and maintains the igniter and pilot flames 47, 54 regardless of changes in the combustion air flow rate while stabilizing the pilot flame 54 and anchoring it to the end of the pilot burner. This assures a continuing, uninterrupted, self-regulating operation of the pilot burner to fully heat up the furnace as quickly as possible.

It is typically preferred to maintain the igniter flame 47 inside hood 42 for the duration of the pilot burner operation so that in the event the main flame generated by the pilot becomes extinguished, it is immediately reignited by the pilot flame.

FIG. 5 schematically illustrates the main pilot flame 54 generated downstream of the pilot burner 36 and its interaction with pilot flame 47 extending from downstream end 68 of the hood. As was earlier described, fuel jets 53 emanating from orifices 52 of pilot nozzle 40 are directed outwardly and away from pilot axis 96 into the furnace interior. To achieve the required heat input, the gaseous fuel jets 53 have velocities which typically range between 500 to 1500 ft./sec. These high velocities also help mix fuel jets with sufficient air to efficiently burn large quantities of fuel gas delivered through the pilot.

In order to assure reliable flame propagation from the flame 47 through the high velocity fuel jets 53, flammable mixtures in substantial parts of the flow immediately adjacent to the nozzle 40 have to be achieved and maintained over the duration necessary to ignite the fuel. This is accomplished by placing orifices 52 about the circumference of the nozzle 40 in two or more staggered rows axially spaced from each other and by the tangential positioning of the orifices spinning off fuel emitted from pilot orifices 52. In each row, the orifices are typically spaced by about one to three times the diameter of the orifices. In a presently preferred embodiment, the spacing between the orifices is approximately twice the nozzle diameter.

Propagation of the flame through gas jets 53 is not sufficient for the flame 54 stabilization. Flow recirculation 58 enhanced by the spinning of fuel emitted from pilot orifices 52 caused by the tangential positioning of the orifices makes the pilot operation efficient and reliable.

As is well known to those skilled in the art, a tangential component imparted to fuel jets to form a forward-directed spiral motion facilitates the formation of gaseous recirculation patterns. The greater the spiral effect, the better the recirculation. The recirculation component of the gas is a function of the so-called “swirl number” S according to the following formula:

$S = \frac{\sum\limits^{\;}\; {{axial}{\mspace{11mu} \;}{flux}\mspace{14mu} {of}\mspace{14mu} {angular}\mspace{14mu} {momentum}}}{{axial}\mspace{14mu} {thrust} \times R}$

-   wherein the axial thrust is the axial force exerted by the     combustion air and gas flows entering the recirculation zone, -   R is the radial distance (from the center of the pilot nozzle) of     pilot orifices 52, and the angular momentum is the rotational force     at R generated by the gas jets 53.

For certain fuels, such as oil, for example, pilot nozzle 40 can extend past the downstream end of main burner 4 into burner throat 26. However, for coal-fired burners, the pilot is recessed into the annular space 34 between coal supply conduit 20 and combustion air conduit 32 to keep the pilot away from the heat, smoke, dust, particulates and the like that are typically present on the interior of coal-fired furnaces, but which are kept out of annular passage 34 and therefore also away from the pilot nozzle by the flow of combustion air.

The combustion of fuel from pilot 36 is continued until the furnace interior has reached the desired temperature, at which time the production fuel, e.g. coal, can be ignited and stably combusted without generating large amounts of pollutants as would occur if combustion were commenced before the required furnace temperature has been reached. 

1. A method of anchoring a high heat output pilot flame to a pilot nozzle associated with a coal burner for industrial furnaces, comprising flowing combustion air along an exterior of a coal supply conduit having a downstream end for discharge of the air into a furnace interior, placing a pilot nozzle proximate to the downstream end of the coal supply conduit, delivering fuel gas to the nozzle through a pipe placed in the combustion air flow, shielding the pipe delivering fuel to the pilot nozzle in the part adjacent to the nozzle with a tubular hood that is open in a downstream direction, while permitting a limited amount of the combustion air to flow into inside the hood, directing a plurality of pilot fuel jets from the pilot nozzle into the furnace interior, discharging at least one igniter fuel jet into an interior of the hood, aspirating air into the hood with the fuel gas jets from at least one of the pilot nozzle or the igniter jet in a quantity to create a flammable mixture inside the hood, igniting the flammable mixture inside the hood to generate an igniter flame that extends past the open downstream end of the hood, and orienting the pilot fuel jets so that they diverge relative to a longitudinal axis of the pilot nozzle and so that the pilot fuel jets have a tangential component relative to the pilot nozzle axis for spinning the fuel about the pilot axis and recirculating a portion of the mixture of fuel with air from the location downstream of the nozzle back to the pilot nozzle.
 2. A method of anchoring a high heat output pilot flame to a pilot nozzle associated with a coal burner for industrial furnaces, the coal burner including a coal supply conduit extending in an axial direction into an interior of the furnace, the method comprising flowing combustion air along an exterior of the coal supply conduit for discharge into the furnace interior, shielding the pilot burner from the combustion air flowing along the exterior of the supply conduit while permitting a limited, controlled amount of air from the combustion air to flow past the pilot nozzle by placing a tubular hood over the pilot nozzle which is open in a downstream direction, discharging at least one high velocity igniter fuel jet into an interior of the hood to thereby lower a pressure inside the hood, using the lower pressure inside the hood to aspirate an amount of air from the combustion air into the hood which depends on the lowered pressure inside the hood, igniting fuel from the igniter jets and air inside the hood to generate an igniter flame that extends past the open downstream end of the hood, directing a plurality of pilot fuel jets from a downstream end portion of the pilot nozzle into the furnace interior, and orienting the pilot fuel jets so that they diverge relative to a longitudinal axis of the pilot nozzle and so that the pilot fuel jets are tangential relative to the pilot nozzle axis for spinning the fuel about the pilot axis and recirculating a portion of the pilot fuel from the pilot fuel jets in an upstream direction toward the pilot nozzle.
 3. A method for preheating a furnace during a warm-up phase of furnace operation, the furnace including at least one main burner having a tubular fuel supply surrounded by a main combustion air duct defining an annular space between the fuel supply and the duct that extends in an axial direction of the main burner, the method comprising positioning a pilot nozzle in the annular space so that the nozzle generally extends in an axial direction of the burner towards an interior of the furnace, discharging readily ignitable fuel jets through orifices in the nozzle oriented toward the interior of the furnace, directing air from the annular space past the nozzle and mixing the air with the fuel discharged from the orifices to form an ignitable mixture, igniting the mixture to form a flame downstream of the nozzle, stabilizing the flame and recirculating portions of the flame and/or the mixture from the furnace interior back towards the nozzle by protecting the air directed past the nozzle from being directly affected by air flowing through the main combustion air conduit, and diverging the fuel jets relative to the axial direction between about 20° to 80° and giving the fuel jets a tangential directional component relative to the axial direction.
 4. A method according to claim 3 including positioning the nozzle outside the furnace interior.
 5. A method according to claim 3 wherein the angle is between about 20° and 80°.
 6. A method according to claim 3 wherein a heat output of the fuel discharged through the orifices is between about 4 and 50 million BTU per hour.
 7. A method according to claim 6 wherein the nozzle comprises a pilot burner, and including limiting a maximum width of the pilot burner transverse to the axial direction to no more than about five inches.
 8. A method according to claim 3 wherein the fluid fuel comprises a gas.
 9. A method according to claim 3 wherein protecting the air comprises placing a tubular hood having an open downstream end about the nozzle, and inhibiting the flow of air from the combustion air duct into the hood with an air flow restrictor positioned proximate an upstream end of the hood.
 10. A method of generating a high BTU output pilot flame during a warm-up phase of operation of a furnace having a main production burner that includes a first conduit for directing coal into an interior of a furnace and a combustion air duct surrounding the conduit defining an annular combustion air passage into the furnace for mixing the coal with combustion air and ignition of the coal, the method comprising placing a pilot nozzle in the combustion air passage so that a downstream of the nozzle is proximate a downstream end of the burner, surrounding the nozzle with a tubular hood having an open downstream end proximate the downstream end of the nozzle and an upstream end, preventing combustion air flowing through the combustion air passage from directly entering the hood through the upstream end thereof while maintaining flow communication between the combustion air passage and an inside of the hood via the upstream end thereof, flowing a pressurized fluid fuel through igniter orifices in the nozzle located inside the tubular hood at a sufficient rate to lower a pressure inside the tubular hood to draw combustion air from the combustion air passage via the upstream end of the hood into the hood, discharging a major portion of the fluid fuel from a plurality of pilot orifices in a downstream end portion of the nozzle, orienting fuel from the plurality of pilot orifices so that fuel jets emitted therefrom angularly diverge in a downstream direction toward the furnace interior and have a tangential flow direction relative to a longitudinal axis of the pilot nozzle, generating an igniter flame that propagates past the downstream end of the hood by igniting the fuel emitted by the igniter orifices inside the tubular hood, and igniting a mixture of fuel from the pilot orifices and combustion air from the combustion air conduit downstream of the main production burner to generate a pilot flame that extends into the furnace interior for heating the furnace interior while portions of the pilot flame and its constituent gases recirculate from the furnace interior rearwardly towards the downstream end of the nozzle while simultaneously spinning relative to the nozzle axis for maintaining a stable pilot flame.
 11. A method according to claim 10 wherein the pilot flame generates a heat output between 5 to 50 million BTU per hour.
 12. A method according to claim 10 wherein preventing combustion air from flowing directly into the hood comprises placing a flow restrictor proximate to and spaced apart from the upstream end of the hood to define a gap between the flow restrictor and the upstream end of the gap through which the combustion air enters the hood.
 13. A method according to claim 12 wherein the flow restrictor is a plate, and varying a width of the gap by moving the plate relative to the upstream end of the hood.
 14. A method according to claim 10 wherein the nozzle includes a fuel supply tube, and wherein the igniter orifices are formed in the fuel supply tube.
 15. Apparatus for preheating a furnace during a warm-up phase of operation and prior to a production phase of operation of the furnace comprising at least one main burner adapted to be extended in an axial direction through a wall of the furnace including a production fuel conduit for directing a production fuel into a furnace interior during the production phase of the furnace, a combustion air duct surrounding the production fuel conduit for flowing combustion air along an annular passage past the main burner into the furnace interior, an elongated pilot fuel nozzle positioned in the air duct arranged substantially parallel to the conduit and the duct and having a transverse extent slightly less than a width of the annular passage, the nozzle including at least one igniter orifice located upstream of a downstream end of the nozzle and a plurality of pilot fuel orifices located proximate the downstream end of the nozzle which angularly diverge in a downstream direction relative to an axis of the nozzle and which are tangentially positioned relative to the nozzle axis, a tubular hood disposed in the annular passage and having an open downstream end proximate the downstream end of the nozzle, a flow inhibitor positioned proximate an upstream end of the tubular hood for preventing combustion air from flowing from the annular passage directly into the hood, and an igniter located inside the tubular hood and proximate the igniter orifices for igniting the fuel emitted by the igniter orifices and generating an igniter flame inside the tubular hood which extends in a downstream direction past the tubular hood for igniting a mixture of fuel emitted by the pilot orifices and combustion air from the annular passage downstream of the tubular hood, whereby the mixture generates a pilot flame downstream of the hood and portions of the pilot flame recirculate from the downstream part of the flame back to the nozzle and spin relative to a longitudinal axis of the nozzle.
 16. Apparatus according to claim 15 wherein the flow inhibitor comprises a plate extending transversely across and axially spaced from the upstream end of the hood to form a gap between the plate and the upstream end of the hood through which air must flow in order to enter the interior of the hood.
 17. Apparatus according to claim 15 wherein the hood has an interior cross-section, and wherein the nozzle has a lesser cross-section than the hood and is positioned adjacent a wall of the hood to define an enlarged space inside the hood where the igniter flame is generated.
 18. Apparatus according to claim 15 wherein the elongated fuel nozzle includes a fuel supply tube, and wherein the igniter orifices are formed in the fuel supply tube. 